Fuel cells use the chemical reaction between hydrogen and oxygen to form water for producing electric energy. The core component of fuel cells is the so-called membrane-electrode assembly (MEA), which is a composite of a proton-conducting membrane and a corresponding gas diffusion electrode (anode or cathode) arranged on both sides of the membrane. The fuel cell is typically formed from a plurality of stacked MEAs, with their electric power being additive. During operation of the fuel cell, hydrogen H2 or a hydrogen-containing gas mixture is supplied to the anode, where an electrochemical oxidation of H2 to H+ takes place by releasing electrons. The protons H+ are transported (bound to water or in an anhydrous environment) from the anode space to the cathode space through the membrane, which separates the reaction spaces in a gas-tight manner and electrically isolates the reaction spaces from one another. The electrons provided at the anode are transported to the cathode via an electrical wire. In addition, oxygen or an oxygen-containing gas mixture is supplied to the cathode, thereby reducing O2 to O2− through combination with the electrons. These oxygen anions react at the same time with the protons transported through the membrane by forming water. With the direct conversion of chemical energy into electrical energy, fuel cells attain a higher efficiency compared to thermal power generators, because they circumvent the Carnot factor.
Each of the electrodes has a catalyst layer facing the membrane. The catalyst layer is disposed on a gas-permeable substrate, the so-called gas diffusion layer (GDL), for homogeneous supply of the reaction gases. The catalyst layer contains reactive centers, typically containing platinum as an effective catalytic component, which is supported on an electrically conducting porous substrate material, for example carbon particles. The reaction centers must satisfy three conditions for efficiently converting the chemical energy of the reaction components. Firstly, the reaction centers of the electrodes must be electrically connected to an external electrical circuit. Secondly, the reaction centers must be connected with the membrane for ion-conduction, so that they can be supplied with protons or discharge protons at a high transport rate. Thirdly, the reaction centers must have ready access to the reaction gases. When all these three conditions are simultaneously satisfied, the so-called 3-phase boundary is formed (solid face=reaction centers of the electrodes//liquid phase=electrolyte//gaseous phase=reaction gases).
Today's most advanced fuel cell technology is based on polymer electrolyte membranes (PEM), wherein the membrane itself is made of an ion-conducting polymer. The most common PEM is based on a sulfonated polytetrafluoroethylene copolymer (trade name: Nafion; copolymer of tetrafluoroethylene and a sulfonyl acid fluoride derivate of perfluor (alkyl vinyl) ether) or of plastic materials analogous to Nafion. The electrolytic conduction takes place via hydrated protons, so that water in the liquid phase must be present for proton conduction. This causes a number of disadvantages. For example, the fuel gases must be humidified during operation of the PEM fuel cell, which increases system complexity. Failure of the humidifier may cause decreased efficiency and irreversible damage to the membrane-electrode assembly. The maximum operating temperature of these fuel cells is limited—also due to a lack of normal long-term stability of the membranes—to 100° C. at ambient pressure (this type of fuel cell is therefore primarily referred to as low temperature PEM fuel cell (LT-PEM fuel cell)). However, for several reasons, operating temperatures above 100° C. are desirable for both mobile and stationary applications. For example, heat transfer to the surroundings increases with increasing temperature difference, which facilitates cooling of the fuel cell stack. The catalytic activity of the electrodes and the tolerance for impurities in the fuel gases also increase with increasing temperature. At the same time, the viscosity of the electrolytic substances decreases with increasing temperature, which enhances material transport to the reactive centers of the electrodes. Finally, the produced product water is in gaseous form at temperatures above 100° C. and can be more readily removed from the reaction zone, so that gas transport paths (pores and meshes) in the gas diffusion layer remain unobstructed, and the electrolytes or electrolyte additives are not washed out.
High-temperature polymer electrolyte membrane fuel cells (HT-PEM- or HTM-fuel cells), which operate at temperatures of 120 to 180° C. and require only little humidification or no humidification at all, have been developed to take advantage of these advantageous properties. The electrolytic conductivity of the membranes used in these second-generation fuel cells is based on liquid electrolytes, in particular acids or bases, bound to the polymer skeleton by electrostatic complex formation which ensures proton conductivity above the boiling point of water even when the membrane is completely dry. For example, U.S. Pat. No. 5,525,436, U.S. Pat. No. 5,716,727, U.S. Pat. No. 5,599,639, WO 01/18894 A, WO 99/04445 A, EP 0 983 134 B, and EP 0 954 544 B describe high-temperature membranes made of polybenzimidazole (PBI) complexed with acids, for example phosphoric acid, sulfuric acid or other acids.
Although conventional HTM fuel cells advantageously have relatively high operating temperatures, they have a problem in that a decrease in the operating temperature below the boiling point of water, for example at the start of the fuel cell or when the system is shut off, can irreversibly damage the MEA, because the produced liquid product water washes out and carries away the electrolyte bound to the membrane, so that there are no longer enough charge carriers available for proton transport. The optimal operating temperature of modern HTM fuel cells is therefore around 160° C. and manufacturers recommend to always maintain the operating temperatures above 120° C., while maintaining the fuel cells at zero current at lower temperatures. However, especially for mobile applications in motor vehicles, a wide temperature window, starting at room temperature and below and reaching temperatures of well above 100° C., is desirable. DE 10 2004 024 844 A and DE 10 2004 024 845 A disclose gas diffusion electrodes for HTM fuel cells which overcome the problem associated with a lack of thermal cycling stability. The catalyst layers of the gas diffusion electrodes are here made of an electrode paste which includes a pore former and a polymer material, wherein the polymer material is preferably made of electrolyte-impregnated polyazoles. The fuel cells with these types of electrodes are significantly more stable against cycling compared to standard electrodes by preventing displacement and loss of the electrolyte. No decrease in the power was observed at a reference temperature of 160° C. when the temperature was cycled between 40 and 160° C. in a two-hour rhythm for more than 800 hours.
In addition to power density and cycle stability, the HTM fuel cells must also satisfy another condition for application in vehicle propulsion, namely cold start ability at low temperatures, ideally at temperatures of about −40° C. Whereas Nafion-based fuel cells can achieve cold start at temperatures around −20° C. with a power density of about 0.05 W/cm2, this has not yet been achieved with HTM fuel cells. In addition, it is desirable to quickly heat the fuel cell to the respective operating temperature, which lies significantly above room temperature for all types of fuel cells. For example, the necessary operating temperature of (Nafion-based) NT-PEM fuel cells is at about 80 to 90° C. and can reach 800 to 1000° C. for so-called solid oxide fuel cells (SOFC). Because the power density of the fuel cells is very low at low temperatures, self-heating due to exothermic fuel cell reactions is insignificant. Accordingly, additional energy must be supplied for heating.
EP 1 351 330 A2 discloses a fuel cell with bipolar plates, wherein particularly the flow channels for the process gases are made at least partially of a material or are coated with a material which forms a hydride in an exothermic hydrogenation reaction, with the generated heat heating the reaction gases and in turn raising the temperature of the MEA. In a preferred embodiment, this internal heating measure is combined with additional external or internal measures, such as electrical heating. However, coating of the bipolar plates with the corresponding material has proven to be complex due to manufacturing difficulties. Moreover, a certain time delay is also observed until the reaction gases cause sufficient heating of the MEA.